1. Field of the Invention
This invention relates to grain drying systems, and particularly to a method and apparatus for controlling low-temperature drying of grain.
2. Description of the Prior Art
Interest in low-temperature grain drying has grown in recent years, partly because of the increasing expense and uncertainty of fuel supplies required for conventional high-temperature continuous-flow dryers. Other factors favoring the low-temperature approach to grain drying include simpler equipment requirements, more efficient use of energy inputs, and higher quality of the conditioned product.
Low-temperature grain drying is similar in concept to drying with natural air. The grain to be dried is stored in a drying/storage bin equipped with a false floor of perpherated metal to permit the passage of air. Ambient air, drawn into the storage bin by way of a fan, is forced up through the wet grain column and passes out of the storage bin through roof vents. As the air passes through the grain, moisture is evaporated from the grain and carried out of the bin. The incoming ambient air provides the primary energy source for removing the moisture from the grain. Generally, low-temperature drying systems include a heater for heating the ambient air a few degrees above ambient to raise the drying capacity of the air.
Low-temperature grain drying is designed for late-fall grain conditioning when low average daily air temperatures restrict mold growth in the slowly drying product. The grain is normally stored in the drying bin and held for spring sale. While the low fall air temperatures restrict spoilage arising from mold growth in the slowly drying product, low-temperature drying depends heavily on the energy in the ambient air for the heat of vaporization required to remove moisture from the grain. Because of the limited capacity of cool air to absorb moisture, the drying process is slow and weather-dependent. For example, operation of the drying fan during periods of high ambient relative humidity adds moisture to the grain already dried. Ventilation of the grain with ambient air at temperatures below 30.degree. F. may result in aggregate freezing of the grain. Operation of the heater during periods of low relative humidity may result in overdrying. Limits on safe storage time, which are governed largely by grain moisture content and temperature, impose restrictions on system operation, and skillful management is required to dry the grain before it spoils.
The use of aeration to maintain the condition of dried or partially dried grain stored over the winter months is accepted practice. The process consists of ventilating the grain with ambient air to limit moisture migration by minimizing temperature gradients and to inhibit mold growth and insect activity by maintaining a low storage temperature. In low-temperature drying systems, aeration is normally accomplished by periodic operation of the drying fan.
During unventilated storage, grain temperatures near the bin wall tend to follow average ambient levels. Temperature gradients develop between the perimeter grain and the grain closer to the center. Convection air currents slowly redistribute moisture from the warmer to the cooler areas. If allowed to continue, this "moisture migration" produces wet-grain zones with high susceptibility to spoilage. Aeration equalizes temperatures within the grain mass and minimizes moisture migration.
Biological activity in stored grain is directly related to grain temperature. Below 50.degree. F., the development of microflora within the grain is restricted significantly. The risk of damage from molds, as well as from stored-grain insects, is reduced greatly by using aeration to maintain low temperatures throughout the bulk. Aeration also serves to remove heat generated by the respiring grain and microorganisms.
While prevailing weather conditions are a major condition in the success of a low-temperature grain drying operation, other influencing factors include the rate of air flow through the storage bin, the grain harvest date, the initial moisture content of the grain at the harvest date, and the temperature of the grain. These variables determine the time required to dry the grain to a given moisture content and the amount of deterioration of grain during the drying period.
Past results have shown that manual operation of the fan and heater based upon general guide lines insures neither optimum drying or conditioning of the grain nor efficient use of energy inputs. As a step in the the analysis of low-temperature grain drying systems, computer models have been designed to simulate the low-temperature drying process. One such computer model is described in an article by P. D. Bloome and G. C. Shore, entitled "Simulation of Low-Temperature Drying of Shelled Corn Leading to Optimization", which appeared in the Transactions of the American Society of Agricultural Engineers, Vol. 15, No. 2, pages 255-265. The computer model was used to simulate the performance of low-temperature drying systems using weather data as an input. Cumulative probability curves were developed to predict successful drying as a function of air flow rate with up to 5.degree. F. of sensible heat added to the input air.
A mathematical model designed to simulate the performance of a temperature-controlled shelled corn storage system was developed and verified experimentally by T. L. Thompson. The model is described in an article entitled "Temporary Storage of Moist Shelled Corn Using Continuous Aeration" which appeared in the Transactions of the American Society of Agricultural Engineers, Vol. 15, No. 2, pages 333-337. The model was used to simulate the effects of air flow rate, harvest date, initial moisture content, grain temperature, and weather conditions on storage deterioration.
Although extensive work has been done on the computer simulation of low-temperature drying processes, very little attention has been directed to actual controls for natural-air and low-temperature drying systems, or to the development of control systems for use in on-site applications.
In one study of natural-air drying of wheat and shelled corn, the effectiveness of continuous ventilation was compared with that of intermittent ventilation under humidistatic control. The intermittent fan was operated only when the relative humidity of the air was below 85 percent. Fan control methods evaluated included continuous operation of the fan; thermostat control, limiting operation to temperatures of 40.degree. F. and below; photocell control, limiting operation to nighttime hours; and manual control, at the discretion of the owner-operator. Another low-temperature drying system employed continuous ventilation and time clock-heater control in which a time clock was programmed to turn off the heater during the hours showing a predicted equilibrium moisture content below a target level.
The development of the control methods referred to above was basically empirical in nature and none of the proposed methods has been entirely satisfactory either because of the need for a considerable amount of operator intervention or because the control method used simply did not result in good conditioning and/or was characterized by inefficient use of energy. Thus, it is apparent that controls to assist management, when present, are limited in scope and effect. A need exists for more comprehensive controls capable of increasing efficiency and reducing management requirements. Such controls are not currently available.